Section 11.3: Voice VLANs

An IP Phone is able to manage certain elements on the manner in which a packet is exposed to a switch. Manner is this instance refers to both user information and voice. Cisco IP Phones have a three port switch: one port connecting to the upstream switch, another connecting to the user's PC, and the third connecting to an internal VoIP data-stream. The user PC and voice ports operate as access mode switch ports while the internal VoIP data-stream port operates as a single VLAN or 802.1Q trunk port. As a trunk, voice traffic is separated from different user data. This enables QoS abilities and enhanced security. When operating as an access link, voice and user data are joined across the single VLAN. This method could have a negative impact on voice quality. The switch can be configured to use either mode.

Voice packets and the QoS data they contain have to be transmitted across a single voice VLAN or over the native VLAN.

11.3.1: Voice VLANs Configuration and Verification

The voice VLAN mode is configured by configuring the switch port where an IP Phone uplink connects. The switch will direct the IP Phone to use the configured mode.

Use the following interface configuration command to specify the voice VLAN mode:

Switch(config-if)# switchport voice vlan { vlan-id | dot1p | untagged | none}

The trunk that is utilized between an IP Phone and a Catalyst switch port is dynamically established. It holds only two VLANs: a voice VLAN and the untagged native VLAN. An active trunk is not displayed in the trunking mode by a Cisco IOS show command. The special trunk is negotiated through DTP and CDP, irrelevant of the trunking mode. STP PortFast is automatically enabled and STP operates in two instances above the trunk.

When a trunk is not utilized, the default for every switch port is none. Modes other than none make use of the special case 802.1Q trunk. The method of encapsulating voice traffic is the only variation between the dot1p and the untagged modes. The dot1p mode places the voice packets on VLAN 0. This mode needs a VLAN ID but not a unique voice VLAN. The untagged mode places the voice packet in the native VLAN. This mode does not need a VLAN ID or a unique voice VLAN.

Because a connection between a switch port and an IP Phone is not displayed in the trunking mode by a Cisco IOS show command, verifying the connection can be a bit complex.

Use the following EXEC command to verify whether the IP Phone has been broadcasted to the switch:

Switch# show cdp neighbors type mod/num detail

Next, use the following EXEC command to verify the access VLAN and voice VLAN (if relevant) used on the switch port:

Switch# show interface type mod/num switchport